1. Field of the Invention
The present invention particularly relates to a transmission modulation apparatus that performs two-point modulation using a PLL (Phase Locked Loop) circuit and thereby generates a RF phase modulation signal, while combines the RF phase modulation signal and an amplitude modulation signal using a high-efficient linear transmission modulator and thereby performs polar modulation, and also relates to a communication apparatus and a mobile wireless apparatus provided with the transmission modulation apparatus.
2. Description of the Related Art
Generally, when a linear transmission modulator is designed, it is necessary to consider a trade-off between efficiency and linearity. Recently, however, technique has been proposed for obtaining both high efficiency and linearity in a linear transmission modulator by using polar modulation for separating an input signal into a phase component and amplitude component, and performing modulation and combining of the phase modulation signal and amplitude modulation signal using a signal of the amplitude component as power supply of a phase modulation amplifier (for example, see “A novel EER transmitter using two-point delta-sigma modulation scheme for WLAN and 3G applications”, IEEE MTT-S 2002 (hereinafter, referred to as Non-patent Document 1).
FIG. 1 is a block diagram illustrating a configuration of a transmission modulation apparatus using conventional polar modulation associated with Non-patent Document 1. In other words, FIG. 1 shows an example of a circuit of the transmission modulation apparatus that performs polar modulation by two-point modulation scheme in a PLL circuit. As shown in FIG. 1, the transmission modulation apparatus is comprised of phase modulation section 100, modulation signal generation section 111 and amplitude modulation section 115a. When a transmission signal is input to modulation signal generation section 111, the signal is separated into a baseband (hereinafter, abbreviated as BB) phase modulation signal and BB amplitude modulation signal. Then, the BB phase modulation signal and BB amplitude modulation signal output from modulation signal generation section 111 are respectively input to phase modulation section 100 and amplitude modulation section 115a. Based on the BB phase modulation signal, phase modulation section 100 phase-modulates a carrier frequency and outputs a RF phase modulation signal. Based on the BB amplitude modulation signal, amplitude modulation section 115a amplitude-modulates the input RF phase modulation signal and outputs a RF modulation signal.
Phase modulation section 100 in FIG. 1 has a configuration of a PLL circuit of two-point modulation scheme to implement wideband phase modulation. In other words, phase modulation section 100 is provided with a PLL circuit comprised of VCO (Voltage controlled Oscillator) 101 that varies the oscillation frequency corresponding to the voltage of a control voltage terminal, frequency divider 102 that divides the frequency of a RF phase modulation signal output from VCO 101, phase comparator 103 that compares phase of an output signal of frequency divider 102 with a phase of a reference signal and outputs a signal corresponding to a phase difference, and loop filter 104 that averages output signals of phase comparator 103, delta-sigma modulator 106 that performs delta-sigma modulation on a BB phase modulation signal to output a dividing ratio to frequency divider 102, D/A converter 107 that converts the BB phase modulation signal into an analog voltage, and filter 108 which suppresses a high-frequency component occurring in D/A converter 107 and outputs a signal to the control voltage terminal of VCO 101.
Here, assuming G(s) as a transfer function of the PLL circuit comprised of VCO 101, frequency divider 102, phase comparator 103, and loop filter 104, BB phase modulation signal φa(s) input to the PLL circuit from A point is multiplied by G(s) that is the transfer function of a low-pass filter. In addition, BB phase modulation signal φb(s) input to the PLL circuit from B point is multiplied by 1−G(s) that is the transfer function of a high-pass filter, where s=jω.
The BB phase modulation signal φ(s) input to the PLL circuit from A point and BB phase modulation signal φ(s) input to the PLL circuit from B point are added at the control voltage terminal of VCO 101, and following equation (1) holds.φ(s)·G(s)+φ(s)·[1−G(s)]=φ(s)  (1)
In other words, the item of transfer function G(s) is canceled, and VCO 101 outputs a RF phase modulation signal unrelated to the transfer function of the PLL circuit. Thus, in the PLL circuit using the two-point modulation scheme, due to low-pass characteristics of the PLL circuit, a BB phase modulation signal is converted into the RF phase modulation signal without undergoing band limitation. By thus performing two-point modulation to generate a RF phase modulation signal, it is possible to implement wideband phase modulation.
The RF phase modulation signal thus generated in phase modulation section 110 is subjected to amplitude modulation in amplitude modulation section 115a. At this point, the BB amplitude modulation signal is multiplexed on the power supply terminal of power amplifier 114 by power control section 113. By this means, power amplifier 114 in amplitude modulation section 115a generates a RF modulation signal including an envelop variation.
Power amplifier 114 of amplitude modulation section 115a operates on the RF phase modulation signal in nonlinear mode (switching mode), while operating on the BB phase modulation signal multiplexed on the power supply terminal in linear mode. Generally, power amplifier 114 operating on a RF input signal in nonlinear mode (switching mode) is high efficient as compared with a power amplifier operating on a RF input signal in linear mode. In the case of thus using polar modulation, the transmitter of linear modulation is capable of using power amplifier 114 that operates in high efficient switching mode, and thereby it is possible to implement a high-efficient linear transmitter.
However, since the BB phase modulation signal and BB amplitude modulation signal separated and output in/from modulation signal generation section 111 are combined again in power amplifier 114, unless the RF phase modulation signal from VCO 101 and BB amplitude modulation signal from modulation signal generation section 111 are input to power amplifier 114 in amplitude modulation section 115a at appropriate timing, there is a possibility of deteriorating performance such as distortion of the spectrum of the RF modulation signal and the like.
Further, in the case of adjusting the timing for inputting the RF phase modulation signal and BB amplitude modulation signal to power amplifier 114, since the communication time is reduced by the adjustment time, it is desired that the time taken for the adjustment is as short as possible.